I wasn't able to be there last year, but I'm planning on attending this year. Glad to see you'll be on the panel again. I really enjoy your blog and look forward to your insight and feedback on the presenting companies.
One clear line that can be drawn between Alpha and Beta in my mind is wether or not your data will persist through to the final release. Previous alpha releases that I've worked on have had a policy that all data and accounts will be wiped clean before entering the beta / general release process.
Not a big deal for traditional 'installed' software, but it's an important factor for web services. Has anyone come across a legitimate site that asked people to play around, provide feedback, but don't expect your data to persist past this release...I've never seen it.